Question re: Water Main Thrust Block Across Sanitary Main

[IanMathers]

I have what may seem to be a foolish question:
We have a situation where parallel mains, 200 mm water and 200 mm sanitary, are to be tapped for a relatively small commercial service connection. I've shown a section detail of the new installation with a 100 mm water line projecting away from the side where the sanitary main is located. My problem is this: the city engineer expresses concern about the concrete thrust block passing above (the sanitary main is 500 mm below) to undisturbed soil at the far side of the existing sanitary trench. Because of the soils conditions, and to keep the 100 mm valve as unobstructed as possible, I hesitate to suggest a deadman thrust block on that side of the tee. Their staff claim that this will cause unspecified difficulties. I really don't see that it will present any challenge that wouldn't be found in a water service line crossing a sanitary main, or the conflicts commonly found between water and sanitary mains in every typical intersection. In 30 years, I’ve done this in the field innumerable times, but until now, the details have never actually shown both water and sewer together. I know you can't fight city hall, but has anyone encountered this sort of objection? How did you resolve it? Considering that almost everything else buried could potentially cross sanitary mains, is it even a valid concern?

 

[fel3]

If you have enough length available along the 100-mm service lateral, I would prefer restrained joints to a thrust block.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[IanMathers]

We do that as standard practice, from a 90 or a tee, 4 joints ea. way are clamped. The thrust block is an added measure.

 

[bimr]

Why throw money away on something that does not work.

Thrust blocks are obsolete because there is no assurance that a) the contractor installed the thrust block correctly; b) there is no assurance that the soils can handle the loading (have you had a geotech evaluate the soils?); c) if someone digs up the pipe in 10 years, how can you be assured that the adjacent pipe held by thrust block are being secured in the dig hole; d) thrust blocks should be installed on undisturbed soil.

Do yourself a favor and forget about the thrust blocks. Use restrained joints.

 

[IanMathers]

Thanks. All your points are valid.

Unfortunately, our city service standards were established many decades ago when development was primarily limited to an area overlain by glacial outwash. Soils ranged from thick clay or silt deposit to sand and gravel. We are also in an area of intermittent permafrost. Current development has extended into surrounding regions of exposed granites (Pre-Cambrian Shield) and the administration insists on using the same methods that were required in entirely different soils conditions. Most of the older staff with field experience has retired and the majority of current bureaucrats landed in administrative positions directly from university.

We still have to satisfy them before development can proceed, and their standards call for both clamping restraints and thrust block.

In this case, they are claiming that there should be no crossing above the sanitary line, that it will make any (unlikely) future maintenance, repair or replacement of the sanitary difficult. Had the site in question been on the other side of the road, we would still have water lines crossing in the same place, at the same elevation, so I don’t understand their position with regard to the thrust block, which they require under their standards.

My question is two-fold; is there something fundamentally wrong with the detail? And if anyone here has encountered a similar circumstance, how was it resolved?

 

[bimr]

The Ten States Standards have a separation requirement that you can reference.

8.8 SEPARATION DISTANCES FROM CONTAMINATION SOURCES

8.8.1 General

The following factors should be considered in providing adequate separation:
a. materials and type of joints for water and sewer pipes,
b. soil conditions,
c. service and branch connections into the water main and sewer line,
d. compensating variations in the horizontal and vertical separations,
e. space for repair and alterations of water and sewer pipes,
f. off-setting of pipes around manholes.

8.8.2 Parallel installation

a. Water mains shall be laid at least 10 feet horizontally from any existing or proposed gravity sanitary or storm sewer, septic tank, or subsoil treatment system. The distance shall be measured edge to edge.
b. In cases where it is not practical to maintain a 10 foot separation, the reviewing authority may allow deviation on a case-by-case basis, if supported by data from the design engineer.

8.8.3 Crossings

a. Water mains crossing sewers shall be laid to provide a minimum vertical distance of 18 inches between the outside of the water main and the outside of the sewer. This shall be the case where the water main is either above or below the sewer with preference to the water main located above the sewer.
b. At crossings, one full length of water pipe shall be located so both joints will be as far from the sewer as possible. Special structural support for the water and sewer pipes may be required.


8.8.4 Exception
When it is impossible to obtain the minimum specified separation distances, the reviewing authority must specifically approve any variance from the requirements of Sections 8.8.2 and

8.8.3. Where sewers are being installed and Section 8.8.2 and 8.8.3 cannot be met, the following methods of installation may be used:

a. Such deviation may allow installation of the water main closer to a sewer, provided that the water main is laid in a separate trench or on an undisturbed earth shelf located on one side of the sewer at such an elevation that the bottom of the water main is at least 18 inches above the top of the gravity sewer.

b. the sewer materials shall be water works grade 150 psi (1.0 Mpa) pressure rated pipe meeting AWWA standards or pipe approved by the reviewing authority and shall be pressure tested to ensure water tightness.

8.8.5 Force mains
There shall be at least a 10 foot horizontal separation between water mains and sanitary sewer force mains. There shall be an 18 inch vertical separation at crossings as required in Section 8.8.3.

8.8.6 Sewer manholes

No water pipe shall pass through or come in contact with any part of a sewer manhole. Water main should be located at least 10 feet from sewer manholes.

8.8.7 Separation of water mains from other sources of contamination

Design engineers should exercise caution when locating water mains at or near certain sites such as sewage treatment plants or industrial complexes. On site waste disposal facility including absorption field must be located and avoided. The engineer must contact the reviewing authority to establish specific design requirements for locating water mains near any source of contamination.

 

[IanMathers]

Thanks for the information. When I worked in the south (Alberta and Saskatchewan) 4 decades ago, we did conform to that range of separation, but there are much different conditions in those jurisdictions. In the north (Northwest Territories), the separation was reduced. Our underground mains are horizontally a minimum of 1.0 m apart, and sanitary is lower by a minimum 0.5 m. Sanitary buried less than 2.0 m, all water lines are insulated DI with 50 mm urethane insulation and a protective outer jacket (used to be double layer CANUSA heat shrink mastic tape, now thin-shell HDPE). Most of the development here in the last 30 years has been pushed into areas of exposed bedrock and all trenches, roads, lot development require extensive blasting. With dwindling accessible sand/gravel deposits, we use 19 mm crushed stone (basically Granular 'A') for bedding and pipe surround and 50 mm crushed to backfill to subbase levels. The cost of trenching and backfill would be even more prohibitive if we were to use greater separations, i.e. separate trenches, although it would avoid my current disagreement with the city.

 

[cvg]

you may have to resolve this by going up the food chain a bit. you really can fight city hall. try the city engineer or the public works director instead of the plan reviewers. perhaps they have suggestion on what would be acceptable. maybe they would be willing to stamp and sign the plans.